Oil and gas deposits tend to occur in geological configurations called traps. Buoyant forces support an oil layer on top of the denser ground water, and similarly a gas layer floats on top of the oil layer. A trap is a geologic configuration that “seals” the hydrocarbon columns in place, preventing their escape. Such escape could result either from fracture of the seal due to hydrocarbon pressure or by capillary seepage through the seal. Such traps often contain commercial deposits of oil or gas. In evaluating such a trap, whether a prospect trap in the course of exploration or a trap of interest in the course of field development, the depths of the gas/oil contact and the oil/water contact are key quantities of interest. These contact depths will depend significantly on the seal capacity, i.e. the ability of the seal to resist fracturing and capillary seepage.
Understanding and predicting total hydrocarbon column height (difference in depth between the hydrocarbon-water contact and the top of the hydrocarbon column) and contacts in a hydrocarbon trap occupies the attention of every hydrocarbon exploration or production company. Seal capacity, which is the maximum hydrocarbon column height a seal can hold before leaking, is typically evaluated on a deterministic basis with little consideration of the substantial uncertainty associated with input parameters. Furthermore, the seal is typically evaluated for either mechanical seal capacity or capillary seal capacity without considering both simultaneously. Also, seal capillary entry pressure, the requisite input parameter for capillary seal capacity analysis, is usually directly measured by mercury injection capillary capacity tests on small pieces of rock. Results from these tests are not readily available everywhere, nor are they necessarily representative of adjacent rocks in the seal.